1. Field of the Invention
The present invention relates to canisters used in fuel vapor processing apparatus for preventing fuel vapor, which may be produced within a fuel tank, from dissipating to the atmosphere. In particular, the present invention relates to canisters having an adsorption chamber, in which an adsorption material for desobably adsorbing fuel vapor and a heat storage material for inhibiting variation in temperature by utilizing latent heat are disposed.
2. Description of the Related Art
A known canister used in a fuel vapor processing apparatus is constructed to adsorb and capture fuel vapor, which may be produced by volatilization of gasoline fuel stored within a fuel tank when a vehicle is stopped. Therefore, the vaporized fuel can be prevented from dissipating to the atmosphere. The known canister includes a tank port communication within an upper portion of the fuel tank, an atmospheric port open into the atmosphere, and a purge port, through which fuel vapor desorbed from the adsorption material flows. If fuel vapor is produced, for example, due to increase in temperature during the operation of a vehicle engine or due to stop of the vehicle, the fuel vapor may be adsorbed by the adsorption material as it flows into the canister from the tank port before being discharged from the atmospheric port. Therefore, the fuel vapor can be prevented from dissipating to the atmosphere. The fuel vapor adsorbed by the adsorption material can be desorbed or purged by the air that may be introduced into the canister via the atmospheric port by a negative pressure produced within an intake manifold of the engine during the operation of the engine or by the operation of a suction pump that is drive-controlled independently of the operation of the engine, so that the adsorption material can be regenerated.
The fuel vapor may be liquefied when adsorbed by the adsorption material within the canister. On the contrary, the fuel vapor may be vaporized when desorbed from the adsorption material. Therefore, when the fuel vapor is adsorbed, condensation heat may be produced to increase the temperature of the adsorption material. In other words, adsorption of the fuel vapor causes an exothermic reaction. On the other hand, when the fuel vapor is desorbed, the temperature of the adsorption material may be decreased due to vaporization heat. In other words, desorption of the fuel vapor causes an endothermic reaction. In general, the adsorption material has a porous structure, and therefore, the adsorption capacity of the adsorption material may increase as the temperature decreases, while the adsorption capacity may decrease as the temperature increases. Therefore, in order to improve the adsorption and adsorption property of the adsorption material, it may be desirous to inhibit the exothermic reaction and the endothermic reaction, which may be caused due to change in phase of the fuel, for inhibiting variation in temperature of the adsorption material.
Japanese Laid-Open Patent Publication No. 2005-233106 teaches a canister having an adsorption chamber in which a heat storage material for inhibiting variation in temperature by utilizing latent heat is disposed together with an adsorption material. The heat storage material is in forms of short cylindrical pellets each having a plurality of microcapsules bonded to each by a binder. Each of the microcapsules contains therein a phase-change material. The adsorption material is granulated also in forms of pellets. The heat storage material pellets and the heat storage material pellets are mixed with each other and filled into the adsorption chamber. With this arrangement, potential increase in temperature of the adsorption material that may be caused when adsorbing the fuel vapor may be inhibited by the latent heat (melting heat) produced when the phase of the phase-change materials contained in the heat storage material changes from a solid phase to a liquid phase. On the other hand, potential decrease in temperature of the adsorption material that may be caused when desorbing the fuel vapor may be inhibited by the latent heat (solidification heat) produced when the phase of the phase-change materials contained in the heat storage material changes from a liquid phase to a solid phase.
Japanese Laid-Open Patent Publication Nos. 63-246462 and 8-4605 teach canisters having adsorption chambers, within which heat storage materials having predetermined shapes are fixedly mounted. In each of these publications, the heat storage material has a heat conductive coefficient and a specific heat (heat capacity) that are higher than those of the heat adsorption material, so that sensible heat can inhibit the temperature variation. More specifically, according to Japanese Laid-Open Patent Publication No. 63-246462, a metal plate (or a plurality of metal plates) primarily made of iron or other metallic material is used as the heat storage material. The metal plate is formed to have a predetermined shape so as to be held between opposite wall surfaces defining the adsorption chamber. In the case of a plurality of the metal plates are used, they are assembled together into a predetermined shape. Because the heat storage material made of metal plate(s) is positioned to intersect with a direction of flow of the fuel vapor (to shield the flow path of the fuel vapor), a plurality of small holes are formed in the heat storage material for ensuring the gas to flow therethrough. According to Japanese Laid-Open Patent Publication No. 8-4605, a spirally wound net made of aluminum is used as the heat storage material and is held within the adsorption chamber.
The effect of the heat storage material for inhibiting the temperature variation of the adsorption material is largely affected by the efficiency of transfer of heat from the adsorption material to the heat storage material. Thus, if the variation in temperature of the adsorption material is not effectively transmitted to the heat storage material, the amount of absorption or dissipation of the latent heat by the heat storage material may be decreased, and therefore, it may not be possible to effectively inhibit the temperature variation. It may be important to always maintain a fixed distance between the adsorption material and the heat storage material for the transmission of heat from the adsorption material to the heat storage material.
However, according to the canister of Japanese Laid-Open Patent Application No. 2005-233106, the heat adsorption material pellets and the heat storage material pellets are mixed with each other and filled to be dispersed into the adsorption chamber. Therefore, the heat storage material pellets may move due to vibrations that may be produced during traveling of the vehicle, and therefore, the heat storage material pellets may be unevenly distributed within the adsorption chamber. This may cause unevenness in the distances between the heat adsorption material pellets and the heat storage material pellets, so that it may not be possible to effectively inhibit the temperature variation. In addition, when unevenness distribution of the heat storage material pellets occurs, the efficiency of inhibiting the temperature variation may differ between different regions within the adsorption chamber. Hence, it is not possible to uniformly inhibit the temperature variation over the entire adsorption material.
In the case of each of the canisters of Japanese Laid-Open Patent Publication Nos. 63-246462 and 8-4605, the heat storage material may not move largely by vibrations produced during traveling of the vehicle because the heat storage material(s) having a predetermined shape is fixedly held within the adsorption chamber, and therefore, the distance between the heat storage material and the adsorption material can be substantially maintained. However, the heat storage material utilizes only sensible heat that is consumed for changing the temperature without accompanying change of phase of material. Therefore, the efficiency for inhibiting the temperature variation is low in comparison with a heat storage material that utilizes latent heat accompanying change of phase of material. In addition, the heat storage materials in these publications are made of metal that is limited in formability, and in particular, not suitable to be formed into a complex configuration, such as a honeycomb configuration. Further, the operation for assembling a plurality of the metal plates of the heat storage material of Publication No. 63-246462 into a predetermined configuration and the operation for spirally winding the metal net of the heat storage material of Publication No. 8-4605 are troublesome and may result in low productivity.
Therefore, there is a need in the art for canisters in which uneven distribution of a heat storage material can be avoided and a distance between the heat storage material and an adsorption material can be substantially fixedly maintained.